1. Field of the Invention
The present invention relates to control systems and, more particularly, to control of rectifiers employing semiconductor devices, such as silicon controlled rectifiers (SCRs). The invention also relates to a method for controlling a rectifier bridge.
2. Background Information
Three-phase rectifier circuits are commonly employed to convert AC signals to DC signals. These circuits often use SCRs disposed in bridge segments, with typically one SCR for each polarity of each AC phase. Typically, a bridge firing control circuit controls the firing point for each rectifier in each AC cycle. Examples of such circuits are disclosed in U.S. Pat. Nos. 5,963,440; 5,963,441; 6,046,917; 6,208,120; and 6,232,751.
It is not uncommon for a plurality of SCR bridges to be operated in parallel with each of the corresponding bridge firing control circuits being controlled by a central firing control circuit. The central firing control circuit manages each of the bridge firing control circuits in order that the corresponding rectifiers in each of the parallel bridges conduct current at the same point in the AC waveform.
SCR bridges are commonly employed in an excitation control system to provide field excitation for a rotating electrical apparatus (e.g., large synchronous generators and motors, utility synchronous generators and motors, industrial synchronous motors and generators, synchronous generators and motors for naval or other shipping applications, synchronous generators and motors for oil well drilling rigs).
A Resistance Temperature Detector (RTD) senses temperature by providing a variable electrical resistance of a metal, which resistance changes with temperature. Platinum is the most commonly used metal for RTDs due to its stability and nearly linear temperature versus resistance relationship. Platinum also has the advantages of chemical inertness, a temperature coefficient of resistance that is suitably large in order to provide readily measurable resistance changes with temperature, and a resistance which does not drastically change with strain. Other types of RTDs include copper, nickel and nickel alloys.
The RTD""s resistance versus temperature relationship is qualified by a term known as xe2x80x9calphaxe2x80x9d. xe2x80x9cAlphaxe2x80x9d is the average percent change in resistance per xc2x0 C. of an RTD between 0xc2x0 C. and 100xc2x0 C. For a 100 xcexa9 platinum RTD, 0.00385xcexa9/xcexa9/xc2x0 C. is the most common alpha. Alpha is also referred to as the temperature coefficient of resistance.
Circuits for sensing temperature from a variable RTD resistance are well known in the art. See, for example, U.S. Pat. Nos. 5,040,724; 6,007,239; and 6,203,191.
Modern excitation control systems typically measure the temperature of a thyristor heat sink assembly. The sensed temperature, in turn, is employed by the excitation control system to determine whether the thyristor is operating within suitable temperature design margins. Typically, a generator alarm or generator trip results when the temperature exceeds a predetermined threshold. The excitation control system may employ active current balance or active temperature balance controls to adjust the current or the temperature between parallel combinations of thyristor bridges. However, known current and temperature balance controls and known excitation control systems continue some level of thyristor gating even if the thyristor temperature is too high.
If a thyristor exceeds its safe operating temperature, then thermal runaway problems may occur. When the thyristor temperature reaches an excessive level, the thyristor fails and causes a short circuit in the thyristor bridge.
Accordingly, there is room for improvement in control systems and methods for controlling rectifier bridges.
These needs and others are met by the present invention, which disables gating of the semiconductor device, such as a thyristor or SCR, before it exceeds a critical temperature. This stops the thyristor or SCR from continuing to heat up and should, therefore, significantly reduce the chance of a short circuit in the rectifier bridge.
The present invention disables gating of the thyristor or SCR that has exceeded a threshold temperature. The temperature employed to compare against a critical threshold temperature may be a temperature of a heat sink for the thyristor or SCR. The temperature may also be thyristor or SCR junction temperature, which is calculated by adding a thermal drop to the measured heat sink temperature.
As one aspect of the invention, a control system comprises: a rectifier including a heat sink having a temperature, and also including a semiconductor device having a gate, the semiconductor device thermally engaging the heat sink; a circuit controlling the rectifier through the gate of the semiconductor device; a circuit measuring the temperature of the heat sink; and means for disabling gating of the semiconductor device based upon the temperature of the heat sink.
The circuit measuring the temperature of the heat sink may include an RTD, which thermally engages the heat sink. The means for disabling gating of the semiconductor device based upon the temperature of the heat sink may comprise means for providing a first signal, which is true when the RTD is not open; means for providing a second signal, which is true when the temperature of the heat sink is greater than a predetermined threshold; and means for disabling gating of the semiconductor device when the first and second signals are both true.
The predetermined threshold may be a first predetermined threshold; and the first signal may be true when the temperature of the heat sink is less than a second predetermined threshold.
The semiconductor device may have a junction and a junction temperature. The means for disabling may include means for measuring current flowing in the semiconductor device; means for calculating the junction temperature based upon the current; and means for disabling gating of the semiconductor device when the junction temperature exceeds a predetermined threshold.
As another aspect of the invention, a method for controlling a rectifier bridge including a heat sink having a temperature, and also including a semiconductor device thermally engaging the heat sink and having a gate, comprises: controlling the rectifier bridge through the gate of the semiconductor device; measuring the temperature of the heat sink; and disabling gating of the semiconductor device based upon the temperature of the heat sink.
The method may further comprise thermally engaging the heat sink with an RTD; measuring a value from the RTD; and determining the temperature of the heat sink from the value.
A first signal, which is true when the RTD is not open, may be provided. A second signal, which is true when the temperature of the heat sink is greater than a predetermined threshold, may be provided. Gating of the semiconductor device may be disabled when the first and second signals are both true.
The method may further comprise employing the semiconductor device having a junction and a junction temperature; calculating the junction temperature; and disabling gating of the semiconductor device when the junction temperature exceeds a predetermined threshold. The method may further comprise thermally engaging the heat sink with an RTD; measuring a value from the RTD; determining the temperature of the heat sink from the value; measuring current flowing in the semiconductor device; calculating heating of the semiconductor device from the current; calculating a temperature rise between the junction and the RTD; and calculating the junction temperature by adding the temperature rise to the temperature of the heat sink.
As another aspect of the invention, a control system comprises: a first rectifier bridge including at least one heat sink having a temperature, and also including at least one semiconductor device, which thermally engages such at least one heat sink and has at least one alternating current input, at least one gate and a direct current output; a second rectifier bridge including at least one heat sink having a temperature, and also including at least one semiconductor device, which thermally engages such at least one heat sink and has at least one alternating current input, at least one gate and the direct current output; a circuit controlling the first and second rectifier bridges through the at least one gate of the at least one semiconductor devices of the first and second rectifier bridges; a circuit measuring the temperature of at least one of the at least one heat sinks; and means for disabling gating of at least one of the at least one semiconductor devices based upon the temperature of a corresponding at least one of the at least one heat sinks.
The circuit measuring the temperature of at least one of the at least one heat sinks may include an RTD, which thermally engages the at least one of the at least one heat sinks and outputs a value. The means for disabling may include means for determining the temperature of the at least one of the at least one heat sinks from the value.
The means for disabling may further include means for providing a first signal, which is true when the RTD is not open; means for providing a second signal, which is true when the temperature of the at least one of the at least one heat sinks is greater than a predetermined threshold; and means for disabling gating of the at least one of the at least one semiconductor devices when the first and second signals are both true.
One of the at least one semiconductor devices may have a junction and a junction temperature. The means for disabling may include means for calculating the junction temperature; and means for disabling gating of the one of the at least one semiconductor devices when the junction temperature exceeds a predetermined threshold.